Glycosylated hemoglobin (HbA1) concentrations were measured in 600 Pima Indians, aged 15 to 91 years, who were participating in the epidemiological studies of diabetes in the Gila River Indian Reservation in Arizona. At the same time plasma glucose levels, fasting and two-hours post-glucose load (75 g), were determined and the plasma glucose values were compared with HbA1 levels in the same population to assess the utility of HbA1 measurements in the diagnosis of diabetes. The distribution of glucose levels in Pimas has been described previously by a model of two overlapping Gaussian curves. Attempts were made to fit the same model to the HbA concentrations. For patients aged 25 years and over, the frequency of HbA1 levels were bimodally distributed and, like the glucose levels, satisfactorily fit the overlapping Gaussian distribution model. These observations indicate the presence of two subpopulations in the Pima Indians, one normal and one diabetic. HbA1 levels in the nondiabetic portion of the pouplation were found to increase as a function of age in the Pimas, as expected. Levels of HbA1 were highly correlated with both fasting and two-hour post-load plasma glucose values, but did not discriminate better than the glucose data between diabetic and nondiabetic subjects.